Attaching blade servers to SANs

Posted on December 19, 2005

—It should come as no surprise that storage is no longer a stand-alone purchase. In fact, storage is now a consideration in almost every IT acquisition. And if blade servers are on your road map for 2006 you will need to consider how to work them into your existing storage infrastructure.

The rate at which end users are connecting blade servers to SANs varies depending on which vendor or industry analyst you ask. If you believe the storage and server vendors, SAN attach rates for blade servers run anywhere between 25% and 50%. The truth, however, may lie somewhere in between.

Richard Villars, vice president of storage systems research at International Data Corp. (IDC), says that approximately 25% to 35% of blade servers ship with Fibre Channel SAN connectivity embedded in the system in the form of a host bus adapter (HBA) or mezzanine card, a low percentage when compared to high-end servers, which tout a 60% to 80% embedded Fibre Channel connectivity rate. But blade servers aren't necessarily designed to compete with servers in the high-end of the market. Taken in context, blades are connected to SANs more frequently than the numbers indicate.

"When you compare blade attach rates to servers that cost less than $25,000, which is where blades play, blades have a significantly higher attach rate to SANs," says Villars.

There is one other market dynamic to consider. Frank Berry, vice president of corporate marketing at QLogic, says blade server technology is still in its relative infancy and major blade server vendors such as IBM, Hewlett-Packard, and Dell are just beginning to roll out embedded Fibre Channel HBAs and blade switches as connectivity options for SANs.

"I think the SAN attach rate is higher than some of the research reports have indicated," says Berry.

SAN connectivity optionsBlade servers, by design, are not fitted with mass storage. Any storage requirements need to be addressed outside of the blade server enclosure and accessed through a network. This presents a seemingly ideal scenario for connecting to SANs, but IT organizations are taking different approaches to connecting blade servers to SANs.

Some vendors and analysts calculate SAN attach rates by the number of SAN switch blades (or blade switches) shipped to customers, but that statistic can be deceiving. Not all blade servers that connect to SANs use switch blades. When a Fibre Channel connection to a SAN is required, blade servers often use a Fibre Channel HBA in combination with a Fibre Channel switch blade or what is called a Fibre Channel pass-through module.

Some blade server vendors such as IBM, Dell and HP are offering embedded pass-through modules into their switch ports as a connectivity option. Fibre Channel pass-through modules receive signals from the blade motherboard and pass them out of the blade enclosure's back-end and ultimately to an existing external Fibre Channel switch. Pass-through modules also tend to be less expensive than switch blades. The modules are optional, and a search of several major vendors' price lists shows that they usually cost less than $200 each.

The benefits to using pass-through modules instead of an internal SAN switch blade include lower cost and increased server density. However, users have to weigh the pros and cons when deciding how to connect their blades to SANs.

On one hand, pass-through modules allow users to connect to SANs via stand-alone Fibre Channel switches external to the blade chassis, without occupying the valuable rack space required by blade switches. There is an inherent cost savings involved with this approach because it allows users to leverage existing switches in their infrastructure. On the other hand, SAN switch blades can aggregate HBA ports on the blade server, which results in fewer ports connecting directly to the SAN, thereby saving users money from a port-count perspective.

Vendors recognize why some users might be hesitant to add another switch to their SAN. Mike Smith, executive vice president of marketing at Emulex, says that employing a switch blade instead of pass-through modules can be more complex. Most organizations already have stand-alone switches in place for connecting devices to the SAN infrastructure, and adding a switch blade brings another inter-switch link into the mix.

SANs are not the only option for blade server storage. Direct-attached storage and NAS have a presence in the blade world as well. Deciding whether a blade chassis should be connected to a SAN is a function of where the servers are deployed and what types of applications they support. Recent survey data from IDC shows that roughly 25% of larger organizations are using NAS as the primary storage resource for their blade servers.

According to Emulex, which manufactures Fibre Channel HBAs and switches that connect blade servers to storage devices and networks, the growth in SAN connectivity for blade servers has been strong and will accelerate with the introduction of 4Gbps Fibre Channel connectivity.

"There is a strong desire among users to integrate blade servers into existing SANs," says Smith. "That's why we're supporting the same device drivers and firmware in our blade products as we are in our traditional server products."

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